This invention relates to a suspension control apparatus for an automotive vehicle, and more particularly, it relates to a suspension control apparatus which can suppress rolling, nose-diving, and vertical motions of a vehicle so as to provide a more comfortable ride.
During the operation of an automotive vehicle, the vehicle is subjected to accelerations in various directions due to braking, turning, changes in speed, and irregularities in the road surface which cause the vehicle body to undergo rolling, pitching, and heaving motions. These motions impart an unpleasant sensation to the passengers of the vehicle. Accordingly, in recent years, there have been proposals for suspension systems which can suppress these motions and provide a more comfortable ride.
For example, Japanese Published Unexamined Utility Model Application No. 61-163710 and Japanese Published Unexamined Patent Application No. 62-34808 disclose suspension mechanisms in which a fluid spring chamber such as an air spring chamber is disposed between the wheels and the body of a vehicle. By controlling the supply of compressed air to the fluid spring chambers, rolling, nose-diving, vertical vibrations, and other motions of the vehicle body can be controlled. For example, during turning, the suspension units on the opposite side of the vehicle from the direction of turning normally attempt to contract, while the suspension units on the same side as the direction of turning attempt to expand, resulting in rolling motion. In order to suppress this motion, a set amount of compressed air is supplied to the fluid spring chambers of the suspension units which tend to contract, and a set amount of compressed air is discharged from the fluid spring chambers of the suspension units which tend to expand. As a result, the vehicle body is tilted back from the direction of tilting caused by the turn, and the vehicle body is maintained horizontal.
In the above-described conventional suspension control systems, in order to compute the magnitude of motions such as rolling, nose-diving, or vertical vibrations of the vehicle, it is necessary to measure the acceleration of the vehicle to the left and right, back and forth, or up and down, respectively. The intake or discharge of compressed air to the fluid spring chambers of the suspension units is controlled in accordance with the detected acceleration. Therefore, it is important that the value of the acceleration which is input from an acceleration sensor be accurate.
The output signal of the acceleration sensor (hereinbelow referred to as a G sensor), which is an analog value, is input to a control apparatus in which it is converted to a digital signal by an A/D converter. Based on the resulting digital signal, actuators such as step motors or high-speed solenoid valves which control the suspension units are operated. However, in the conventional suspension control systems, the G sensor is independently grounded with respect to the control apparatus, so the drive current for the above-mentioned actuators flows only into the ground of the control apparatus, and the ground potential of the control apparatus rises with respect to the ground potential of the G sensor. As a result, the voltage of the G sensor output signal as seen from the ground circuit of the A/D converter of the control apparatus appears smaller by the amount that the ground potential of the control apparatus has increased, the G sensor output signal is incorrectly judged as having changed, a signal to make the actuators operate is mistakenly output, and incorrect control is performed.